That seems to be the position taken in a lawsuit filed by the Municipal Employees Association of San Diego. From the U-T:
The Municipal Employees Association, San Diego’s largest public employee union representing white-collar workers, has accused Mayor Jerry Sanders of violating state labor laws by refusing to negotiate the elements in the initiative while at the same time using the power of his public office to generate public support for it.
If successful, the complaint, filed last week with the state Public Employment Relations Board, could prevent the initiative from appearing on the ballot and essentially nullifies the nearly 116,000 signatures collected to trigger a public vote.
Like that last little bit? 116,000 citizens sign a ballot initiative, but because the mayor used his first amendment rights to support the effort, its invalid, according to the unions. To be clear, the labor law cited does nothing to prohibit citizen initiatives, and they are protected by the state constitution. In most states, I would be unfazed, wondering how fast the lawsuit would be tossed. Unfortunately, we live in California, where the rule of law appears more tenuous.